I just read through the Fiendish Codexes for the first time ever (boo!), and I'm thinking about the demonic possession rules a lot. They're incredibly open-ended, and can only lead to hilarity.
Did you know you can possess only part of an object, like that section of wall, or this one word on a whole page? And did you know you can make it so everybody coming into contact with that object gets cursed? You do remember that line from the Bestow Curse spell, "You may also invent your own curse, but it should be no more powerful than those described above", right? Possess the ground (or the air) right in front of the city gates, and curse everybody touching it to immediately empty their bladder, or grope the next person, or whatever you feel like.
Did you know there's no minimal size for an object you possess? Yes, you can possess an atom if you wish! Try sundering that, adamantine-Greatsword-wielding Barbarian! Also, possess an oxygen atom (not a molecule! That would be stupid!) and be inhaled by whomever, riding around in their bloodstream for a while, eventually being built right into their body's molecular structure. Are you now possessing the inhaler, with no save allowed? I sure don't know!
If you have sufficiently high HD and Cha, you can make your possessed object move of its own accord as if it were an Animated Object. You can also hide inside it to pass through warded areas and escape Divination spells. How about possessing a big, spiky ball of adamantine (preferably subject to a prior Hardening spell, of course), and just bowling everybody over? Nobody's ever going to find out just why that metal ball is doing what it's doing (if you have a good Hide score, but skills are easy to boost), or how to stop it!
Be creative, be a pest, spread the chaos of the Abyss! All you need is be a demon of some sorts. The Possession ability even only increases CR, not LA!